Modeling a 3 Blade Boat Propeller
Here's how I would create a propeller like the one pictured.

First create a cylinder and extrude 3 of the faces out and model them into a propeller blade shape.  To get decent topology you can extrude along the edge of the blade and then use Grid Fill to fill it in.



Then just add a Simple Deform modifier and set it to Twist, I used an angle of 90°.



Note: in the above example, instead of modeling all three fins at once, I deleted a third of the cylinder and used an Array modifier with the pivot point set to an empty rotated 120°.  That way I only had to model one blade.